1. Field of the Invention
This invention relates to a liquid crystal display device and a method for manufacture of a liquid crystal display device. In particular, this invention relates to a liquid crystal display device which utilizes a state in which liquid crystal molecules are aligned vertically with no voltage applied, and to a method for manufacture of same.
2. Description of the Related Art
In the prior art, as liquid crystal displays (LCDs) using an active matrix, TN-mode liquid crystal display devices in which liquid crystal material having a positive dielectric constant anisotropy is aligned parallel to the substrate surfaces, and with a 90° twist between the opposing substrates, have been widely employed. However, there is the problem that viewing-angle characteristics are poor when using the TN mode, and so various studies have been conducted in order to improve viewing-angle characteristics.
As an alternative method, the MVA (Multidomain Vertical Alignment) method was developed, in which a liquid crystal material having a negative dielectric constant anisotropy is aligned vertically, and protrusions on the substrate surface and slits in electrodes regulate the direction of inclination of liquid crystal molecules when a voltage is applied; and this method has been successful in greatly improving viewing-angle characteristics.
MVA-method liquid crystal panels are explained using the examples of FIG. 1A, FIG. 1B, and FIG. 2. FIG. 1A and FIG. 1B are schematic perspective views showing the alignment of liquid crystal molecules in the liquid crystal panel of an MVA-method liquid crystal display device; FIG. 2 is a schematic plan view showing the alignment direction of liquid crystal molecules in the liquid crystal panel of an MVA-method liquid crystal display device.
In the liquid crystal panel of this MVA-method liquid crystal display device, the liquid crystal molecules 1 with a negative dielectric constant anisotropy which are between the two glass substrates are aligned vertically as shown in FIG. 1A when no voltage is applied. Pixel electrodes connected to TFTs (thin film transistors, not shown) are formed on one of the glass substrates 2, and a counter electrode is formed on the other glass substrate 3. Uneven portions 4 are formed in alternation on the pixel electrodes and on the counter electrode.
When a TFT is in the off state, that is, when no voltage is applied, liquid crystal molecules are aligned in the direction vertical to the substrate interface, as shown in FIG. 1A. When the TFT is put into the on state, that is, when a voltage is applied, the influence of the electric field causes the liquid crystal molecules to be inclined in the horizontal direction, and due to the structures of the uneven portions, the direction of inclination of the liquid crystal molecules 1 is regulated. As a result the liquid crystal molecules are aligned in a plurality of directions within one pixel, as shown in FIG. 1B. For example, when uneven portions 4 are formed as in FIG. 2, liquid crystal molecules 1 are aligned in each of the directions A, B, C and D. Thus in an MVA-method liquid crystal display device, with a TFT in the on state the liquid crystal molecules are aligned in a plurality of directions, and so satisfactory viewing-angle characteristics are obtained.
In the above MVA method, an alignment control film does not regulate the direction of inclination of liquid crystal molecules. Hence alignment treating processes such as rubbing, which is almost essential for horizontal-alignment methods of which TN is representative, are unnecessary. In terms of processes, this eliminates the problems of static electricity and impurities resulting from rubbing, and so also obviates the need for cleaning processes after alignment processing. Moreover, the problem of irregularities due to scattering in pretilt angles in relation with alignment is also eliminated, and there are also the advantages that processes can be simplified and yields improved, enabling cost reductions.
However, the MVA method requires that an alignment control film itself be installed; and such problems as reduced liquid crystal panel quality due to thickness irregularities when printing the alignment control film, lower yields due to inclusion of foreign matter, the complexity of equipment and processes, increased facility costs, increased material expenses, extended tact times, and other problems of cost increasing due to printing, cannot be avoided. Further, although techniques are known (see for example Japanese Unexamined Patent Application Publication No. 11-95221 (scope of claims)) for the coexistence of cured resins and other polymers which regulate the liquid crystal alignment in the liquid crystal layer, incorporation of such techniques has not advanced to a level at which use of alignment control films has become unnecessary.
Further, while there has been rapid advance toward ultra-large motherglass for the substrates of liquid crystal panels, it is getting more and more difficult to provide equipment to print alignment control film that can accommodate such ultra-large motherglass.
Also, the use of equipment for printing alignment control films is limited to planar shapes, owing to the difficulty in handling film-shape substrates which are thin and flexible.